Typically fertilizer granules are coated in a rotating drum by spraying and/or dribbling coating components onto the top surface of the bed of fertilizer granules. In this method, the coating materials, such as castor oil and isocyanate, do not easily penetrate into the granule bed. One of the reasons for this is their high viscosity and surface tension, which inhibits penetration into the granule bed. Because these coating materials do not readily penetrate into the bed, they first coat the drum surface, and are subsequently transferred onto the surface of the fertilizer granules. In this two stage process, the coating materials cannot spread effectively on the surface of the fertilizer granules within the time it takes for the castor oil and isocyanate to react to form polyurethane (approximately four minutes at 75° C.). A similar problem exists when other coating materials, such as other thermoplastic polymers or thermoset polymers, are used. Accordingly, cured or dried coating material builds up on the drum surface. This coating build up on the drum surface is called “fouling”.
In prior art processes, fouling is severe, especially on the drum surface around the coating material nozzles. In many cases, the coating drum must be cleaned every two weeks. Not only is the cleaning process expensive, but the production interruptions and higher raw material consumption negatively affects net profits. Moreover, fouling adversely affects the quality of the controlled release product.
Coating quality (and therefore product performance) is reduced in prior art processes since there is limited opportunity for coating materials to mix in the stoichiometric ratios necessary to form the desired optimum coat on substrate granules. For instance, isocyanate is typically dribbled onto the surface of the granules in a narrow line, while castor oil is either dribbled or sprayed. In either case, the opportunity for mixing of these two components with each other and on the surface of the granules is limited, even if the isocyanate and castor oil are delivered substantially simultaneously. One reason for this is that granules moving in a rotating drum exhibit a linear layer flow, with slow lateral mixing between layers.
Attempts have been made to improve the coating process in a rotating drum. One approach has been to dip injector nozzles into the fertilizer bed so coating components are injected into the fertilizer bed immediately below the surface of the bed. (See, for example, U.S. Pat. No. 5,374,292; U.S. Pat. No. 5,547,486; U.S. Pat. No. 5,858,094; and U.S. Pat. No. 6,537,611.) However, there still remains a need in the art to further improve mixing and reduce fouling to increase the efficiency of the coating process.